1. Field of the Invention
The present invention relates to an X-ray imaging apparatus having a plurality of pixels arranged on a detection surface, for directly converting an irradiated incident X-ray distribution to an image signal, and more particularly to an X-ray imaging apparatus having a diode, i.e., a two terminal element having a non-linear resistance characteristic, made by thin film technology. The present invention also relates to an X-ray imaging analysis apparatus provided with the X-ray imaging apparatus for any number of uses, including medical and industrial use.
2. Discussion of the Background
Conventional imaging systems, used mainly for an X-ray image analysis, typically include an X-ray imaging screen film system using a screen film made of silver salt, a computed radiography (hereinafter called CR) system using an imaging plate coated with a photostimulable phosphor, and a digital radiography (hereinafter called the DR) system using a combination of an image intensifier (I.I.) for converting x-rays into visible light and a television set to produce an X-ray image (I.I.--TV system).
On the other hand, a clinical laboratory requires an X-ray imaging apparatus having the following features: (1) a capability of recognizing whether a correct part was photographed (imaged) by displaying a photographed part immediately after photographing, (2) a capability of processing an image for quantitative analysis, (3) high efficiency without need for developing a film, and (4) a capability of storing, detecting, conveying and handling digitizing image data. Thus, recent advances in X-ray imaging produce digital images for further processing.
In order to digitize a silver salt screen film photographed by a conventional X-ray imaging analysis, scanners have to be operated after developing a photographed film. However, this is an inefficient use of resources.
The CR system uses an imaging plate coated with a photostimulable phosphor instead of a silver salt film as an X-ray detector. The imaging plate has a very wide dynamic range as compared with the silver salt screen film and can make an image with a wide range of X-ray doses.
Upon irradiation of an X-ray on the imaging plate, an electron energy level of a luminescent center included in the photostimulable phosphor such as Eu is heightened by the X-ray energy to such an extent that an X-ray intensity distribution is stored as a latent image. Then, the luminescent center is excited by scanning the imaging plate with infrared radiation or a red laser light (wavelength .lambda..sub.1), and a resultant energy is output as light having a wavelength .lambda..sub.2, where .lambda..sub.2 &lt;.lambda..sub.1 while the luminescent center heightened by the X-ray energy returns to a reference level. An electric signal in proportion to the X-ray intensity distribution can be obtained by collecting the light having the wavelength .lambda..sub.2 and amplifying the collected light in a photomultiplier tube.
The CR system using a conventional imaging plate, however, is noisy and low in resolution. Also, it is easily damaged, the performance thereof deteriorates with use, and it is expensive.
In the DR system using a combination of an image intensifier and a television set, an area of the X-ray input surface of the image intensifier defines the size that can be imaged and is equivalent to a maximum field of view of about 16 inches. At an X-ray input section of the image intensifier, light from phosphor irradiated by X-rays is focused at a photoelectric surface. A photoelectron irradiated from the photoelectric surface is accelerated in an electric field of the image intensifier and then output to a phosphor screen which is illuminated. Thus, an image having variable density at the X-ray input section can be amplified and displayed at an output screen. This output image is picked up by a television camera through an optical system producing an electric image.
In the case of imaging an area of lungs, however, an imaging area 40 cm.times.40 cm is necessary. Further, a very fine image intensifier having a large diameter, high precision optical apparatus, and a high resolution TV camera are also necessary, increasing the volume of the entire apparatus.
To resolve some of the problems described above, another X-ray imaging apparatus is constructed of a two-dimensional array (hereinafter called the X-ray plane detector), formed by amorphous-silicon (hereinafter called the a-Si) thin film transistors (hereinafter called TFTs), and photoelectric transfer elements which are made via a production method of a liquid crystal panel as recommended in U.S. Pat. No. 4,689,487. FIG. 11 illustrates the structure of an X-ray imaging analysis apparatus having the X-ray plane detector and FIG. 12 illustrates the structure of the X-ray plane detector, respectively.
In FIG. 11, an X-ray analysis apparatus 901 is shown that includes an X-ray tube 103 supplied from a high voltage generating circuit (not shown) that confronts a detected object P, an X-ray plane detector 107 also confronting the detected object P, the X-ray tube 103 and the X-ray plane detector 107 sandwiching the detected object P, an analog-digital converter 113 for converting analog image signal output from the X-ray plane detector 107 to a digital image signal, an image processor 115, an image recording device 117 such as an optical disc, a digital-analog (D/A) converter 119, and an image monitor device 121.
The X-ray plane detector 107 includes a fluorescent board 105a for converting X-rays transmitted through the detected object P to an optical image and a plane detector 105b located adjacent to the fluorescent board 105a.
An operation of the conventional system will now be explained. As shown in FIG. 11, the detected object P is located adjacent to the fluorescent board 105a through a proper light shield member (not shown). X-rays transmitted from the X-ray tube 103 irradiate the object P and X-rays transmitted through the object form a transmitted X-ray image of the detected object P, which is converted to an optical image by the fluorescent board 105a. The optical image is converted to an image signal by the plane detector 105b on which photoelectric transfer elements are arranged in a two-dimensional array.
An analog image signal output from the X-ray plane detector 107 is converted to a digital image signal by the A/D converter 113 and input to the image processor 115. The image processor 115 processes the digital image signal by various methods and stores necessary image data in the image recording device 117. The digital image signal output from the image processor 115 is converted to an analog image signal by the D/A converter 119 and displayed on a screen of the image monitor device 121.
The X-ray plane detector 107 is a photoelectric transfer type and as shown in FIG. 12 includes picture elements e(h, k) (1.ltoreq.h.ltoreq.2000, 1.ltoreq.k.ltoreq.2000) which are arranged in a two dimensional array (hereinafter called a TFT array) and in a square shape, for example, and 2,000 pixels are arranged along one edge of the square, a select circuit for selecting a pixel along vertical and horizontal directions, and an amplifier.
Each pixel e(h, k) is formed in a photoelectric transfer film 140 that includes a PN junction formed by an a-Si Thin-Film Transistor (TFT) 144 and a-Si photo diode 148 and a pixel capacitor 142 (hereinafter called a Cst). A common bias voltage of minus several volts, for example, is applied to a P-side terminal of each photoelectric transfer film 140 by an electric source 148.
The a-Si TFT 144 included in each pixel e(h, k) has a source terminal connected to an N-side terminal of an a-Si photodiode 148, a drain terminal connected to a signal line S(h), and a gate terminal connected to a scanning line G(k).
The scanning line G(k) is alternatively controlled ON-OFF by a horizontal scanning shift register 152. The other terminal of the signal line S(h) is connected to an input of an amplifier 154 for detecting a signal through a vertical switching switch 146. The vertical switching switch 146 is provided corresponding to the signal line S(h) and alternatively controlled ON-OFF by the vertical scanning shift register 150. A signal of the selected signal line is connected to an input of the amplifier 154. The image signal is obtained from the output of the amplifier 154.
An operation of the X-ray plane detector 107 will next be explained. After X-ray exposure, light is irradiated from the fluorescent board 105a to the plane detector 105b. Light striking the plane detector causes optical current flow in the photoelectric transfer film of each pixel e(h,k), and electric charge is stored in each respective Cst in proportion to an intensity of the irradiated light.
Then, the scanning lines for alternatively selecting a pixel line are driven by a scanning line driving circuit 152. When the all TFTs connected to one scanning line G(k) are switched on, the electric charge stored in the each Cst is transferred to the switching switch 146 through the signal line S(h).
The switch 146 inputs the electric charge corresponding to each sequentially selected pixel to the amplifier 154 to convert the electric charge to a point sequential signal capable of display on a CRT. The amount of electric charge corresponds to the irradiated light intensity and the output amplitude of the amplifier is changed accordingly.
In the system as shown in FIG. 12, a digital image can be obtained directly by A/D converting the output signal of the amplifier 154. A pixel area as shown in FIG. 12 has structure similar to a TFT-LCD utilized in a notebook type personal computer and can be easily produced as a thin and large scale model.
The above explanation relates to an indirect conversion type X-ray plane detector in which irradiated X-rays are converted to visible radiation with phosphors or the like and the visible radiation is converted to electric charge with a photoelectric transfer film corresponding to each pixel.
In addition, there is a direct conversion type X-ray plane detector for directly converting irradiated X-rays to electric charge. In the direct conversion type X-ray plane detector, material and film-thickness of X-ray electric converting film have to be changed as compared with the indirect conversion type. An amount of bias voltage applied to the X-ray (optical) electric converting film and the applied method thereof have to be changed as well.
In the indirect conversion type, a potential of minus several volts is applied to a photoelectric transfer film being in part a capacitance Cse and having a 1.about.2 .mu.m thickness. When radiation irradiates the photoelectric transfer film, an electric charge generated in the photoelectric transfer film is divided by voltage division and partially stored in the juxtaposed pixel capacitance Cst. In this case, the voltage applied to Cst is several volts at most, similar to a bias voltage applied to the photoelectric transfer film. In this case, the voltage applied to the Cst is several volts at maximum, similar to a bias voltage applied to the photoelectric transfer film.
On the other hand, in the direct conversion type, an X-ray electric charge converting film having a thickness of 500 .mu.m.about.1 mm and a Cst are connected in series and several kV is applied. When X-rays are irradiated to pixels of the X-ray electric charge converting film, an electric charge produced in the X-ray electric converting film is stored in the Cst.
However, when excessive X-rays are irradiated to pixels of the X-ray electric charge converting film, image quality is lowered because electric current leakage of a TFT reading switch is increased, or because charge leaks into an adjacent pixel in the X-ray electric charge converting film. Also, excessive X-rays can cause the TFT reading switch and insulating layer of the Cst to be broken. Therefore, in the direct conversion type, it is necessary to avoid applying high voltage to the Cst.
In an over-voltage protection technique of the conventional direct conversion type X-ray plane detector, the prior art (Denny L. Lee et al., SPIE, vol. 2432, pp 237, 1995) shows a dielectric layer insulator formed on a x-ray electric charge converting film as shown in FIGS. 13 and 14. Three condensers, a condenser in the dielectric layer (Cd), a condenser in the X-ray electric charge converting layer (Cse), and a condenser in a pixel electrode (Cst) are connected in series and the produced electric charge is divided and stored in the X-ray electric charge transfer film so that the TFT and Cst insulator can be protected from breakage and the image can be protected from lowered quality.
In the conventional over-voltage protection technique taught by Denny Lee et al., however, the structure described needs more time to reset Cd after picking up the image than a minimum amount of time required to capture a moving image in real time. Therefore, if a detected object is moving, the transmitted x-ray image can not be seen in real time.
On the other hand, Japanese Patent Application No. 8-161977 shows a TFT actuated as a clip diode (hereinafter referred to as a protection diode) with respect to each pixel. This prior art reference does not teach provision of capacitors connected in series, unlike Denny et al., so that a reset time is sufficiently short and an X-ray transmitted image of a moving detected object can be seen in real time. However, the TFT is used as a protection diode. If one TFT is used, a ratio of the area occupied by the protection TFT to the pixel area is relatively large. Further, if leakage current is controlled, a plurality of TFTs is necessary in some cases. As the result, the number of TFTs per pixel is increased, an area ratio of pixel electrode (the effective sensor area) per pixel area (hereinafter called an opening ratio) is not easily made to be common for each pixel and the requisite pixel capacity Cst is not obtained easily.
Further, if a PN junction diode is used as a protection diode, a number of production steps is increased, and a yield of the diodes becomes lower. Therefore, production cost would be remarkably increased, since the production steps of the protection diode are different from those of the conventional a-Si TFT array. In addition, the PN junction diode made of a-Si has a poor rectification characteristic and substantial leakage current.